BR112020023655B1 - Process for preparing a multimodal ethylene copolymer in suspension in a cascade of reactors and apparatus for polymerizing olefinic monomers in suspension in a cascade of reactors - Google Patents

Abstract

"SUSPENSION PROCESS TO PREPARE ETHYLENE COPOLYMERS IN A REACTOR CASCADE". This is a process for preparing a multimodal ethylene copolymer in suspension in a cascade of reactors comprising a first polymerization reactor and one or more subsequent polymerization reactors, which comprises separating the suspension formed in the cascade. of multimodal ethylene copolymer particulate reactors and recovered suspension medium, purifying a portion of the recovered suspension medium in a purification section to produce purified components of the recovered suspension medium, and recycling at least some or a portion of the components purified from the recovered suspension medium to the first polymerization reactor of the reactor cascade, wherein the purified components of the recovered suspension medium recycled to the first polymerization reactor, which comprise the diluent, undergo catalytic hydrogenation before being introduced in the first polymerization reactor, and an apparatus for polymerizing olefinic monomers in suspension in a cascade of reactors.

Description

Field of invention

[0001] The present disclosure relates to a process for preparing a multimodal ethylene copolymer in suspension in a reactor cascade. The present disclosure specifically relates to a process for preparing a multimodal ethylene copolymer suspended in a reactor cascade in which a suspension of multimodal ethylene copolymer particles formed in the reactor cascade is transferred to a separator and separated into particles. of multimodal ethylene copolymer and recovered suspension medium, wherein a portion of the recovered suspension medium is purified in a purification section to produce the purified components of the recovered suspension medium, and at least some or a portion of the purified components of the medium recovered suspension is recycled to the first polymerization reactor of the reactor cascade.

Background of the invention

[0002] Processes for preparing ethylene copolymers in suspension in a reactor cascade are recognized methods for producing ethylene polymers and, for example, disclosed in document EP 0 905 152 A1 or in document WO 2012/028591 A1. Such processes make it possible to define different reaction conditions in the polymerization reactors and thus produce different polymer compositions in the individual polymerization reactors. Consequently, produced multimodal ethylene copolymers are characterized by having, for example, a good combination of product properties and processability. Suspension processes for preparing ethylene polymers commonly employ a hydrocarbon or hydrocarbon mixture as a diluent. The suspending medium, which forms the liquid or supercritical phase of the suspension, however, comprises, in addition to the diluent as the main component, additional components as well, such as dissolved ethylene, comonomers, aluminum alkyls, and hydrogen and dissolved reaction products, such as oligomers and waxes. The principles for producing multimodal ethylene copolymer in suspension in a reactor cascade are, for example, disclosed in F. Alt et al., Macromol. Symp. 2001, 163, 135 to 143.

[0003] It is known that the polymer properties of the produced ethylene copolymers not only depend on parameters such as molecular weight distribution or comonomer content, but also on the incorporated comonomer. With increasing comonomer chain length, product properties such as film performance or environmental stress crack resistance (ESCR) commonly increase. However, ethylene copolymers that have a relatively short chain comonomer, such as ethylene/1-butene copolymers, are still commercially important polymers.

[0004] Because they are capable of operating a commercially successful ethylene polymerization process, it is necessary that unreacted monomers that are discharged from the polymerization reactor, together with the polymer produced, be recycled into the polymerization process. In suspension polymerization processes, the other components of the suspension medium also need to be recycled in the most economical way possible. Therefore, it is an option to directly recycle the suspension medium without separating the components. However, to produce multimodal ethylene copolymers that have good mechanical properties, such as a high ESCR, it is generally necessary that one of the polymerizations in one of the polymerization reactors is an ethylene homopolymerization. This means that all streams feeding such a polymerization reactor must be free of comonomers. A common method for removing unwanted components from a liquid mixture composition is distillation. However, in order to achieve effective separation through distillation in an economical manner, the boiling points of the components must be sufficiently far apart.

[0005] Consequently, common technologies for preparing ethylene polymers in suspension employ specific combinations of comonomer and diluent. Commercially used combinations are, for example, 1-hexene as comonomer and isobutane as diluent or 1-butene as comonomer and hexane as diluent. However, these technologies do not allow the incorporation of comonomers that have a boiling point very similar to the boiling point of the diluent to produce ethylene copolymers that have an ethylene homopolymer component in a polyethylene production facility under, respectively, economic conditions. favorable.

[0006] Consequently, there is a need to overcome the disadvantages of the prior art and to provide a process, which allows the production of multimodal ethylene copolymers that have a good combination of product properties and processability in a cascade of reactors with ethylene comonomers. any boiling point and which, however, allows economical recycling of the components of the suspension medium to the reactors of the reactor cascade.

Summary of the invention

[0007] The present disclosure relates to a process for preparing a multimodal ethylene copolymer in suspension in a reactor cascade comprising a first polymerization reactor and one or more subsequent polymerization reactors comprising, polymerizing, in the reactor cascade , ethylene and one or more C3-C12-1- alkenes at temperatures of 40 to 150 °C and pressures of 0.1 to 20 MPa in the presence of a polymerization catalyst and form a suspension of multimodal ethylene copolymer particles in a suspending medium comprising a diluent, transfer the multimodal ethylene copolymer particle suspension to a separator, in which the suspension is separated into multimodal ethylene copolymer particles and recovered suspending medium, purify a portion of the recovered suspending medium into a purification section for producing purified components of the recovered suspension medium, and recycling at least some or a portion of the purified components of the recovered suspension medium to the first polymerization reactor of the reactor cascade, wherein the purified components of the recovered suspension medium recovered recycled to the first polymerization reactor, which comprise the diluent, undergoes catalytic hydrogenation before being introduced into the first polymerization reactor.

[0008] In some embodiments, the purified components of the recovered suspension medium, which are recycled to the first polymerization reactor, amount to 5 to 70% by weight of the recovered suspension medium.

[0009] In some embodiments, an ethylene homopolymer is prepared in the first polymerization reactor and an ethylene copolymer is prepared in a subsequent polymerization reactor.

[0010] In some embodiments, the difference between the normal boiling point of one of the comonomers used to feed the reactor cascade and the normal boiling point of the diluent or, if the diluent is a mixture of components, the difference between the normal boiling point of one of the comonomers used to feed the reactor cascade and the initial normal boiling point or final normal boiling point of the diluent is not greater than 15 °C, with normal boiling points being defined as boiling points at 1,013.25 hPa.

[0011] In some embodiments, the multimodal ethylene copolymer is an ethylene-1-hexene copolymer.

[0012] In some embodiments, the multimodal ethylene copolymer comprises at least two comonomers.

[0013] The multimodal ethylene copolymer comprises at least 1-hexene and 1-butene as comonomers.

[0014] In some embodiments, a portion of the recovered suspension medium is recycled directly to a subsequent polymerization reactor.

[0015] In some embodiments, the process for producing the purified portion of the recovered suspension medium includes a step of evaporating a portion of the recovered suspension medium and then recondensing the evaporated portion of the suspension medium.

[0016] In some embodiments, the process for producing the purified portion of the recovered suspension medium includes a distillation step.

[0017] In some embodiments, the process for producing the purified components of the recovered suspension medium includes a wax removal step.

[0018] In some embodiments, the suspension of polyethylene particles removed from the first polymerization reactor of the reactor cascade is used to feed a separator, with a portion of the suspension medium being separated from the suspension and recycled to the first polymerization reactor. of the reactor cascade, and a concentrated suspension of polyethylene particles is transferred to the next polymerization reactor of the reactor cascade.

[0019] In some embodiments, the reactor cascade comprises at least 3 polymerization reactors and the suspension of polyethylene particles taken from the second polymerization reactor is used to feed a separator, with a portion of the suspension medium being separated from the suspension and recycled to the second polymerization reactor, and a concentrated suspension of polyethylene particles is transferred to the third polymerization reactor.

[0020] In some embodiments, the present disclosure provides an apparatus for polymerizing olefinic monomers in suspension in a cascade of reactors comprising: - at least two polymerization reactors connected in series that form a cascade of reactors, - a separator for separating the suspension in polyolefin particles and in a recovered suspension medium, - transfer lines for transferring a suspension of polyolefin particles in a suspension medium from one polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade and from the last polymerization reactor of the reactor cascade to the separator, and - a recycling line for recycling parts of the recovered suspension medium to the first polymerization reactor of the reactor cascade, the recycling line for recycling parts of the suspended medium Suspension recovered for the first polymerization reactor of the reactor cascade is equipped with a catalytic hydrogenation unit.

[0021] In some embodiments of the apparatus, the transfer line for transferring the suspension of polyolefin particles from the first polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade is equipped with a separator for separating a part of the suspension medium of the suspension transferred from the first polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade and wherein the apparatus further comprises a recycling line for recycling the suspension medium separated from the suspension in the separator installed between the first polymerization reactor of the reactor cascade and the next polymerization reactor of the reactor cascade to the first polymerization reactor of the reactor cascade.

Brief description of the drawings

[0022] Figure 1 schematically shows a configuration for preparing multimodal ethylene copolymers according to the process of the present invention.

Detailed description of the invention

[0023] The present disclosure relates to a process for preparing a multimodal ethylene copolymer in suspension in a reactor cascade. The term “multimodal” hereby refers to the modality of the ethylene copolymer obtained and indicates that the ethylene copolymer comprises at least two polymer fractions that are obtained under different reaction conditions, regardless of whether this modality can be recognized as separate maxima on a gel permeation chromatography (GPC) curve or not. Different polymerization conditions can, for example, be achieved by using different concentrations of hydrogen and/or by using different concentrations of comonomer in different polymerization reactors. The term “multimodal” as used herein shall also include “bimodal.”

[0024] Ethylene copolymers are prepared by polymerizing ethylene and one or more C3-C12-1-alkenes in the presence of a polymerization catalyst. C3-C12-1-alkenes can be linear or branched. Preferred C3-C12-1-alkenes are linear C3-C10-1-alkenes, such as propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene or 1-decene, or C2-C10- 1-branched alkenes, such as 4-methyl-1-pentene. It is also possible to polymerize ethylene with mixtures of two or more C3-C12-1-alkenes. Preferred comonomers are C3-C8-1-alkenes, particularly 1-butene, 1-pentene, 1-hexene, 1-heptene and/or 1-octene. The amount of units in the prepared multimodal ethylene copolymers derived from the incorporated comonomers is preferably 0.01% by weight to 25% by weight; more preferably, from 0.05% by weight to 15% by weight and particularly from 0.1% by weight to 12% by weight. Specific preference is given to processes in which ethylene is copolymerized with from 0.1% by weight to 12% by weight 1-hexene and/or 1-butene and specifically from 0.1% by weight to 12% by weight. % by weight of 1-hexene.

[0025] In a preferred embodiment of the present disclosure, the multimodal ethylene copolymer is an ethylene-1-hexene copolymer, that is, it is an ethylene copolymer that was obtained through the copolymerization of ethylene as the main monomer and 1-hexene as a comonomer.

[0026] In another preferred embodiment of the present disclosure, the multimodal ethylene copolymer comprises at least two comonomers, that is, the multimodal ethylene copolymer is a terpolymer or a copolymer that comprises more than two types of comonomers. Specifically, preferred ethylene copolymers comprise at least 1-hexene and 1-butene as comonomers.

[0027] Polymerization can be carried out using all common olefin polymerization catalysts. This means that polymerization can be carried out, for example, with the use of chromium oxide-based Phillips catalysts, with the use of titanium-based Ziegler - or Ziegler-Natta catalysts, with the use of single-site catalysts or with the use of mixtures of such catalysts. For the purposes of the present disclosure, single-site catalysts are catalysts based on chemically uniform transition metal coordination compounds. Additionally, it is also possible to use mixtures of two or more of these catalysts for the polymerization of olefins. Such mixed catalysts are often designed as hybrid catalysts. The preparation and use of such catalysts for olefin polymerization are generally known.

[0028] The preferred catalysts are of the Ziegler type, which preferably comprise a titanium or vanadium compound, a magnesium compound and, optionally, an electron donating compound and/or a particulate inorganic oxide as a support material.

[0029] Ziegler-type catalysts are normally polymerized in the presence of a cocatalyst. Preferred cocatalysts are organometallic compounds of metals of Groups 1, 2, 12, 13 or 14 of the Periodic Table of Elements, in particular, organometallic compounds of Group 13 metals and, especially, organoaluminum compounds. Preferred cocatalysts are, for example, organometallic alkyls, organometallic alkoxides or organometallic halides.

[0030] Preferred organometallic compounds include lithium alkyls, magnesium or zinc alkyls, magnesium alkyl halides, aluminum alkyls, silicon alkyls, silicon alkoxides and silicon alkyl halides. More preferably, the organometallic compounds comprise aluminum alkyls and magnesium alkyls. Even more preferably, the organometallic compounds comprise aluminum alkyls, more preferably trialkylaluminum compounds or compounds of that type in which an alkyl group is replaced by a halogen atom, for example by chlorine or bromine. Examples of such aluminum alkyls are trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-hexylaluminum or diethylaluminum chloride or mixtures thereof.

[0031] The process of the present disclosure comprises a polymerization that occurs in a medium, the so-called suspension medium, which is in a liquid or supercritical state under the conditions in the respective polymerization reactor and in which the ethylene polymer produced is insoluble and forms solid particles. The solids content of the suspension is generally in the range of 10 to 80% by weight, preferably in the range of 20 to 40% by weight.

[0032] The suspension medium, which forms the liquid or supercritical phase of the suspension, commonly comprises, as the main component, a diluent, but also comprises additional components such as, for example, dissolved comonomers or monomers, dissolved cocatalysts or sequestrants, such as aluminum alkyls, dissolved reaction aids such as hydrogen or dissolved reaction products of the polymerization reaction such as oligomers or waxes. Suitable diluents must be inert, that is, they must not decompose under reaction conditions. Such diluents are, for example, hydrocarbons having from 3 to 12 carbon atoms and, in particular, saturated hydrocarbons such as isobutane, butane, propane, isopentane, pentane, hexane or octane, or a mixture thereof. In a preferred embodiment, the diluent is a hydrocarbon mixture. To produce hydrocarbon mixtures from a raw material, a smaller demand is required to separate the components of the raw material than to produce a specific hydrocarbon, and, accordingly, hydrocarbon mixtures are more economically attractive as a diluent, but have the same diluent performance as specific hydrocarbons. However, hydrocarbon mixtures can have a range of boiling points.

[0033] When performing a copolymerization of ethylene with a comonomer, which has a boiling point that is close to the boiling point of the diluent or that is within a boiling point range of the diluent, a separation of the comonomer from the diluent through evaporation or distillation is not the most economical way possible. When using a mixture of components as diluent, this could even imply a necessary separation of the diluent into high-boiling components and low-boiling components. Consequently, the process of the present disclosure is specifically suitable when the boiling points of the diluent and one of the comonomers are close or overlap, because the process allows recycling the diluent to a polymerization reactor, in which a homopolymerization of ethylene is carried out without separating the comonomer from the diluent.

[0034] Therefore, in a preferred embodiment of the present disclosure, the difference between the normal boiling point of one of the comonomers used to feed the reactor cascade and the normal boiling point of the diluent or, if the diluent is a mixture of components , the difference between the normal boiling point of one of the comonomers used to feed the reactor cascade and the initial normal boiling point or the final normal boiling point of the diluent is not greater than 15 °C, preferably not greater than than 10 °C, and particularly not greater than 6 °C, when normal boiling points are defined as boiling points at 1013.25 hPa.

[0035] The process of the present disclosure can be carried out using all industrially known suspension polymerization processes at temperatures in the range of 40 to 150 °C, preferably 50 to 130 °C and particularly preferably 60 to 90 °C, and under pressures of 0.1 to 20 MPa and particularly and preferably of 0.3 to 5 MPa. Processes of this type are generally known to those skilled in the art.

[0036] The process of the present disclosure is carried out in a reactor cascade of at least two polymerization reactors that are connected in series. The reactor cascade comprises a first polymerization reactor and one or more subsequent polymerization reactors. These reactors are not restricted to any specific design, preferably these reactors are, however, cycle reactors or stirred tank reactors. It is possible that the process of the present invention can only be carried out in a series of two reactors, a first polymerization reactor and a second polymerization reactor as a subsequent reactor. However, it is also possible that there are two or more subsequent polymerization reactors arranged downstream of the first polymerization reactor. Preferably, different polymerization conditions are established in each of the polymerization reactors. The reactor cascade may also comprise one or more additional polymerization reactors, such as prepolymerization reactors upstream of the first polymerization reactor.

[0037] In a preferred embodiment of the process of the present disclosure, the ethylene homopolymer is prepared in the first polymerization reactor and an ethylene copolymer is prepared in a subsequent polymerization reactor. In order to be able to prepare an ethylene homopolymer in the first polymerization reactor, no comonomer is used to feed the first polymerization reactor, either directly or as a component of a feed stream or a recycle stream that is introduced into the first polymerization reactor. polymerization of the reactor cascade. If the reactor cascade comprises one or more prepolymerization reactors, the prepolymerization is preferably carried out without the addition of comonomers.

[0038] The polymerization reactors of the process of the present disclosure are, preferably, single reactors. However, it is also possible, specifically for the first polymerization reactor, to have two or more reactors, arranged in parallel or in series, to operate under identical or substantially identical conditions and to act as the first polymerization reactor of the process of the present invention. revelation.

[0039] In a preferred embodiment of the present disclosure, the multimodal ethylene copolymer of the present disclosure is prepared in a series of a first reactor and a subsequent polymerization reactor, wherein the polyethylene prepared in the first polymerization reactor is an ethylene homopolymer , preferably a low molecular weight ethylene homopolymer, and the polyethylene prepared in the subsequent polymerization reactor is an ethylene copolymer, preferably a high molecular weight copolymer. The multimodal ethylene copolymers thus obtained preferably comprise from 35 to 65% by weight of ethylene homopolymer prepared in the first polymerization reactor and from 35 to 65% by weight of ethylene copolymer prepared in the subsequent polymerization reactor.

[0040] In another preferred embodiment of the present disclosure, the multimodal ethylene copolymer of the present disclosure is prepared in a series of three polymerization reactors, that is, in a first polymerization reactor and in two subsequent polymerization reactors, in which the polyethylene prepared in the first polymerization reactor is an ethylene homopolymer, preferably a low molecular weight ethylene homopolymer, the polyethylene prepared in one of the subsequent polymerization reactors is an ethylene copolymer, preferably a high molecular weight copolymer, and the polyethylene prepared in another subsequent polymerization reactor is an ethylene copolymer with a high molecular weight, preferably an ultra-high molecular weight copolymer. The multimodal ethylene copolymers thus obtained preferably comprise 30 to 60% by weight; more preferably, from 45 to 55% by weight of ethylene homopolymer prepared in the first polymerization reactor, from 30 to 65% by weight; more preferably, from 20 to 40% by weight of ethylene copolymer prepared in a subsequent polymerization reactor, and from 1 to 30% by weight; more preferably, from 15 to 30% by weight of high molecular weight ethylene copolymer prepared in the other subsequent polymerization reactor.

[0041] In the process of the present disclosure, the suspension of multimodal ethylene copolymer particles formed in the reactor cascade is transferred to a separator, in which the multimodal ethylene copolymer particles are separated from the suspension medium. This separation into particles of multimodal ethylene copolymer and recovered suspension medium can be carried out in all suitable separation apparatus, such as centrifuges, decanters, filters or combinations thereof. Preferably, the separator is a centrifuge. In a preferred embodiment of the present disclosure, the suspension withdrawn from the reactor cascade is first transferred to a separator feed vessel and transported from the separator feed vessel to the separator.

[0042] Typically, the recovered suspension medium comprises more than 80% by weight of diluent. Additional components of the recovered suspension medium are ethylene, comonomers, aluminum alkyls, and hydrogen, and dissolved reaction products such as oligomers and waxes.

[0043] Preferably, the majority of the recovered suspension medium is recycled to the reactor cascade. Preferably, all polymerization reactors of the reactor cascade are provided with recycled parts of the recovered suspension medium. Preferably, from 90 to 99.99% by weight; more preferably, from 95 to 99.5% by weight, and particularly from 98 to 99% by weight of the recovered suspension medium is recycled to the reactor cascade. Those parts of the recovered suspension medium that are not recycled to the reactor cascade include, for example, continuous pump washes, flue gas that may be vented to purge gaseous impurities from the feed streams, or gaseous byproducts of the polymerization process, or dissolved reaction products, such as waxes, which are intentionally removed from the recovered suspension medium.

[0044] The recovered suspension medium is preferably first used to feed a suspension medium collection container and then to be recycled to the polymerization reactors of the reactor cascade, with the recovered suspension medium being removed from the container collection of suspension medium.

[0045] The multimodal ethylene copolymer particles obtained in the separator are commonly still wet and preferably have a suspension medium content in the range of 15% by weight to 40% by weight and, more preferably, 20% by weight to 35% by weight. The separated multimodal ethylene copolymer particles are, in accordance with preference, used to feed a two-stage drying section in which residual suspension medium is removed from the multimodal ethylene copolymer particles with hot nitrogen in closed cycles. The dried multimodal ethylene copolymer particles are preferably pneumatically conveyed to an extrusion section where appropriate amounts of additives are added and the mixture is melted, homogenized and pelletized. Preferably, most of the components of the suspension medium, which are separated from the multimodal ethylene copolymer particles in the drying process, are collected and recycled to the reactor cascade.

[0046] In the process of the present disclosure, a portion of the recovered suspension medium is purified in a purification section to produce purified components of the recovered suspension medium. Purification, in the context of the present disclosure, means that a composition is separated into one or more separate compositions or that one or more components of a composition are removed from the composition and a composition is purified with no component removed (or components removed), or at least substantially devoid of it, is obtained. A purification can, however, go so far that the individual components of the composition are isolated. Such a purification process may, for example, include recovered suspending medium removal components, which have a significantly lower boiling point than that of the diluent, from the recovered suspending medium, and/or oil removal components. suspension medium, which has a significantly higher boiling point than that of the diluent, for example oligomers or waxes, from the recovered suspension medium. The amount of recovered suspension medium passing through the purification section is preferably 1 to 90% by weight; more preferably, from 5 to 80% by weight of the recovered suspension medium recycled to the reactor cascade. In a preferred embodiment of the purification process, the recovered suspension medium is separated into two or more components that are recycled to the reactor cascade in individual recycling loops. After separation, each of the individual recycling circuits may additionally comprise purification steps. The components of the recovered suspension medium, which can be recycled to the reactor cascade in the individual recycling circuits, can be, in addition to the diluent, ethylene and comonomers. The purified components of the recovered suspension medium can be transferred to any of the reactors in the reactor cascade.

[0047] Preferably, the process for producing the purified components of the recovered suspension medium includes a step of evaporating a portion of the recovered suspension medium and then recondensing the evaporated portion of the suspension medium. Typically, the evaporated portion of the recovered suspension medium comprises ethylene, hydrogen, those comonomers, which have a lower or similar boiling point than that of the diluent used, and parts of the diluent. This means, when, for example, n-hexane or a mixture of hexane isomers is used as a diluent and 1-butene is used as a comonomer, the majority of the 1-butene comprised in the recovered suspension medium forms a part of the evaporated part of the suspension medium. Preferably, the majority of the evaporated portion of the recovered suspension medium is recycled to one or more of the polymerization reactors in individual recycling loops; most preferably, after having gone through one or more additional purification steps.

[0048] In a particularly preferred embodiment of the present disclosure, the process for producing the purified components of the recovered suspension medium includes a distillation step. Preferably, the lower boiling point component obtained through distillation comprises ethylene, hydrogen, those comonomers, which have a lower or similar boiling point than that of the diluent used, and parts of the diluent. This means that when, for example, n-hexane or a mixture of hexane isomers is used as a diluent and 1-butene is used as a comonomer, the majority of the 1-butene comprised in the recovered suspension medium forms a part of the suspension component. lowest boiling point obtained through distillation. Preferably, the majority of the lower boiling component obtained through distillation is recycled to one or more of the polymerization reactors in individual recycling circuits; most preferably, after having gone through one or more additional purification steps.

[0049] Preferably, the component with the highest boiling point obtained through distillation comprises the majority of the diluent and those comonomers, which have a boiling point greater than or similar to that of the diluent used. Preferably, the majority of the higher boiling component obtained through distillation is recycled to one or more of the polymerization reactors; most preferably, after having gone through one or more additional purification steps.

[0050] Preferably, the process for producing the purified components of the suspension medium recovered in the purification section includes a wax removal step. The wax removed from the polymerization process can be burned to generate energy or can be sold as a byproduct of the polymerization process.

[0051] The process for producing the purified components of the recovered suspension medium may further comprise additional purification steps, such as, for example, a purification by means of adsorption, a purification by means of absorption or a purification by means of a membrane purification process.

[0052] In the process of the present disclosure, at least some or a portion of the purified components of the recovered suspension medium are recycled to the first polymerization reactor of the reactor cascade. The components of the recovered suspension medium that are preferably recycled to the first polymerization reactor of the reactor cascade are diluent and ethylene. These components are preferably recycled in individual recycling circuits.

[0053] Preferably, 5 to 70% by weight of the recovered suspension medium is recycled as purified components of the recovered suspension medium to the first polymerization reactor of the reactor cascade; more preferably, from 10 to 60% by weight and particularly from 15 to 50% by weight of the recovered suspension medium is recycled to the first polymerization reactor of the reactor cascade.

[0054] According to the process of the present disclosure, the purified components of the recovered suspension medium recycled to the first polymerization reactor, which comprise the diluent, undergo catalytic hydrogenation before being introduced into the first polymerization reactor. A catalytic hydrogenation is a chemical reaction in which unsaturated chemical compounds are reacted with hydrogen in the presence of a hydrogenation catalyst. The unsaturated chemical compounds that may be contained in the suspension medium for recycling to the first polymerization reactor may be various chemical compounds that are introduced into the reactor cascade as impurities from the feed streams or that have been formed in process-side reactions. polymerization. However, the main parts of the unsaturated chemical compounds, which are hydrogenated in accordance with the present disclosure, are remaining ethylene and, predominantly, the comonomer or comonomers.

[0055] Catalytic hydrogenation can be carried out in the liquid phase or in the gas phase and is, preferably, carried out in the liquid phase.

[0056] As hydrogenation catalysts, the use can be produced from the usual catalysts for hydrogenation, for example, catalysts based on platinum, palladium, rhodium or transition metals such as molybdenum, tungsten, chromium or iron, cobalt, copper and nickel, which can be used individually or in combination, generally applied to supports such as activated carbon, ceramics, etc. Hydrogenation is generally carried out from 50 to 300 °C, preferably from 100 to 250 °C.

[0057] Examples of hydrogenation catalysts for hydrogenations in the gas phase are compositions based on palladium or platinum, particularly preferably platinum or palladium in alumina. Examples of hydrogenation catalysts for method hydrogenations in the liquid phase are nickel or cobalt-based catalysts activated by trialkyaluminum, such as cobalt (acetylacetonate) or nickel (octanoate); rhodium catalysts, such as Wilkinson catalyst (Rh(PPh3)3Cl); ruthenium catalysts such as Ru(H)Cl(PPh3)3. Alternatively, heterogeneous platinum, platinum oxide or palladium catalysts can be used as a suspension in the reaction medium. Hydrogenation catalysts that can be used for methods 1) and 2) are described in “Catalytic Hydrogenation” (R.L. Augustine, Dekker Publishers, New York, 1965) and in “Advanced Organic Chemistry”, 4th Edition, p. 771 to 780 (J. March, publisher Wiley, New York, 1992).

[0058] In a preferred embodiment of the present disclosure, a portion of the recovered suspension medium is recycled directly to a subsequent polymerization reactor. Preferably, all subsequent polymerization reactors of the reactor cascade are provided with directly recycled parts of the recovered suspension medium. This means that the parts of the recovered suspension medium are preferably recycled to the second reactor and the optional subsequent additional polymerization reactor, further comprising, in addition to the diluent, unreacted comonomers and ethylene, cocatalysts or sequestrants, such as aluminum alkyls, dissolved reaction aids such as hydrogen and dissolved reaction products of the polymerization reaction such as oligomers or waxes. Direct recycling of the separate suspension medium saves the effort of working the materials and allows the reuse of the contained cocatalyst and comonomer or comonomers, thus reducing overall operating costs. Preferably, from 10 to 99% by weight and, more preferably, from 20 to 95% by weight of the recovered suspension medium is recycled directly to the reactor cascade.

[0059] Preferably, the fresh diluent that is introduced into the reactor cascade to replace diluent losses is not directly used to feed one of the polymerization reactors, but is used to feed a component of the purification section or is added to part of the recovered suspension medium that is transferred to the purification section.

[0060] The process of the present disclosure allows to produce multimodal ethylene copolymers that have a good combination of product properties and processability in a cascade of reactors under economically favorable conditions, since the recovered suspension medium is recycled to the first polymerization reactor of the The reactor cascade is free of comonomers and thus it is possible to produce an ethylene homopolymer in the first polymerization reactor and, in the meantime, use recycled components of the suspension medium to form main parts of the suspension medium in the first polymerization reactor.

[0061] In a preferred embodiment of the present disclosure, the suspension of polyethylene particles removed from the first polymerization reactor of the reactor cascade is used to feed a separator, with a portion of the suspension medium being separated from the suspension and recycled to the first polymerization reactor of the reactor cascade, and a concentrated suspension of polyethylene particles is transferred to the next polymerization reactor of the reactor cascade. Separators suitable for separating a portion of the suspension medium from the suspension removed from the first polymerization reactor may be centrifuges, filters, cyclones, concentrators (thickeners) or combinations thereof. By recycling a portion of the suspension medium from the suspension withdrawn from the first polymerization reactor directly back to the first polymerization reactor, the amount of hydrogenated components of the recovered suspension medium that are required to replace the suspension withdrawn from the first polymerization reactor is reduced and, consequently, the operational costs for such a process are reduced.

[0062] In another preferred embodiment of the present disclosure, the reactor cascade comprises at least three polymerization reactors and not only does the suspension transferred from the first polymerization reactor to the second polymerization reactor pass through a separator, but also the suspension transferred from the second polymerization reactor to the third polymerization reactor passes through a separator. By recycling a portion of the suspension medium from the suspension taken from the second polymerization reactor directly back to the second polymerization reactor, the amount of suspension medium transferred from the second to the third polymerization reactor is minimized and, consequently, the amount of comonomer transferred from the second to the third.

[0063] The polymerization reactor is minimized. Such an approach is preferred for the use of different comonomers in the second and third polymerization reactors or for the use of comonomer mixtures of a different composition in the second and third polymerization reactors. An example of the same could be the use, mainly, of 1-butene as a comonomer in the second polymerization reactor and the use, mainly, of 1-hexene as a comonomer in the third polymerization reactor or vice versa.

[0064] Figure 1 schematically shows a configuration for preparing multimodal ethylene copolymers, according to a process of the present disclosure, in which polymerization occurs in a series of three reactors.

[0065] To polymerize the olefins in a first suspension polymerization reactor (1), purified recycled components from a recovered suspension medium are used to feed the reactor (1) through a feed line (2). The other components of the reaction mixture, such as catalyst, ethylene, possible comonomers, and polymerization aids, are used to feed the reactor through one or more feed lines (3). As a result of polymerization in the reactor (1), a suspension of solid polyethylene particles in a suspension medium is formed. This suspension is used to feed, through line (4), a separator (5) in which the suspension formed in the reactor (1) is separated into a concentrated suspension, which is transferred through line (6) to a second polymerization reactor (7), and a liquid suspension medium that is recycled to the first polymerization reactor (1) via the line (8).

[0066] Inside the polymerization reactor (7), polymerization also occurs. The directly recycled recovered suspension medium is used to feed the reactor (7) via lines (21), (22) and (24) and the purified recycled components of the recovered suspension medium can be used to feed the reactor (7 ) through lines (41) and (24). Fresh ethylene, comonomer or additional components of the reaction mixture can be used to feed the reactor (7) through one or more feed lines (9). The reactor suspension (7) is thereafter used to feed, via the line (10), a third polymerization reactor (11) in which further polymerization is carried out. The directly recycled recovered suspension medium is used to feed the reactor (11) through lines (21) and (23) and the purified recycled components of the recovered suspension medium can be used to feed the reactor (11) through the lines (41) and (22). One or more feed lines (12) allow supplemental feeding with ethylene, comonomer or additional components of the reactor reaction mixture (11).

[0067] The suspension of solid particles of multimodal ethylene copolymer formed in the reactor (11) is continuously transferred through the line (13) to a feed container of the separator (14). The suspension is then transported via line (15) to the centrifuge (16), where the suspension is separated into solid particles of multimodal ethylene copolymer and recovered liquid suspension medium. The isolated multimodal ethylene copolymer particles are conveyed via line (17) to a dryer (not shown) and thereafter to a pelletizing unit (not shown).

[0068] The recovered suspension medium is transferred via the line (18) to a suspension medium collection container (19). From there, the recovered suspension medium can be recycled via pump (20) through lines (21) and (22) and lines (23) and (24) to the polymerization reactor (7) and/or or to the polymerization reactor (11).

[0069] A portion of the recovered suspension medium is branched out of line (21) and transferred via line (25) to a purification section comprising an evaporator (26), a distillation column (34) and a adsorbent unit (37). The recovered suspension medium branched out through line (21) is conveyed to the evaporator (26). The higher boiling point fractions of the recovered suspension medium are withdrawn from the bottom of the evaporator (26) through the line (27) and transferred to a wax separator container (28). Liquid wax is withdrawn from the bottom of the wax separator container (28) via line (29) and transported to an incineration unit (not shown), e.g. for steam generation, or to a solidification unit (not shown) to be sold. A gaseous fraction is removed from the top of the wax separator container (28) through the line (30), which passes through the heat exchanger (31) to be condensed and sent through the line (25) back to the evaporator (26).

[0070] The fractions of the recovered suspension medium that are evaporated in the evaporator (26) are removed from the upper part of the evaporator (26), through the line (32) that passes through the heat exchanger (33), to be condensed and transferred to a distillation column (34). Low boiling point components are removed from the upper part of the distillation column (34) through line (35). When operating polymerization in reactors (1), (7) and (11) with 1-butene as comonomer, 1-butene is withdrawn through line (35) and transferred to a 1-butene recovery unit (not shown ) to purify the 1-butene and to recycle the 1-butene to one or more of the polymerization reactors of the reactor cascade.

[0071] The lower stream from the distillation column (34) is transferred through the line (36) to an adsorbent unit (37) to remove polar impurities. After passing through the adsorbent unit (37), part of the lower current to be recycled to the polymerization reactors (7) and (11) is transferred via the line (38) to a lower current collection container (39). . From there, the lower stream is recycled, through the pump (40), through the line (41) and the lines (23) and (24) to the polymerization reactor (7) and/or to the polymerization reactor (7). polymerization (11).

[0072] The part of the lower stream to be recycled to the first polymerization reactor (1), is transferred, after passing through the adsorbent unit (37), through the line (42) to a catalytic hydrogenation unit (43) . The hydrogenated bottom stream is transferred via line (44) to a hydrogenated bottom stream collection container (45). From there, the lower hydrogenated stream is recycled, through the pump (46), through the line (2) to the first polymerization reactor (1).

[0073] To replace diluent losses, for example, by discharging the diluent together with the multimodal ethylene copolymer particles and not completely recovering the diluent in the drying process of the multimodal ethylene copolymer particles, the fresh diluent is used to feed the purification section through line (47).

[0074] The present disclosure additionally provides an apparatus for polymerizing olefinic monomers in suspension in a cascade of reactors comprising: - at least two polymerization reactors connected in series forming a cascade of reactors, - a separator for separating the suspension in polyolefin particles and in a recovered suspension medium, - transfer lines for transferring a suspension of polyolefin particles in a suspension medium from one polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade and from last polymerization reactor of the reactor cascade to the separator, and - a recycling line for recycling parts of the recovered suspension medium to the first polymerization reactor of the reactor cascade, the recycling line for recycling parts of the suspended medium recovered to the first polymerization reactor of the reactor cascade is equipped with a catalytic hydrogenation unit.

[0075] Preferably, the transfer line for transferring the suspension of polyolefin particles from the first polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade is equipped with a separator for separating a portion of the medium from suspension of the suspension transferred from the first polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade and wherein the apparatus further comprises a recycling line for recycling the suspension medium separated from the suspension in the separator installed between the first polymerization reactor of the reactor cascade and the next polymerization reactor of the reactor cascade to the first polymerization reactor of the reactor cascade.

Claims (15)

1. Process for preparing a multimodal ethylene copolymer in suspension in a reactor cascade comprising a first polymerization reactor and one or more subsequent polymerization reactors, characterized by the fact that it comprises, polymerizing, in the reactor cascade, ethylene and one or more C3-C12-1-alkenes at temperatures of 40 to 150°C and pressures of 0.1 to 20 MPa in the presence of a polymerization catalyst and form a suspension of multimodal ethylene copolymer particles in a suspension medium comprising a diluent, transferring the multimodal ethylene copolymer particle suspension to a separator, in which the suspension is separated into multimodal ethylene copolymer particles and recovered suspension medium, purifying a portion of the recovered suspension medium in a purification to produce purified components of the recovered suspension medium, and recycling at least some or a portion of the purified components of the recovered suspension medium to the first polymerization reactor of the reactor cascade, wherein the purified components of the recovered suspension medium are recycled to the first polymerization reactor, which comprises the diluent, undergoes catalytic hydrogenation before being introduced into the first polymerization reactor. 2. Process according to claim 1, characterized by the fact that the purified components of the recovered suspension medium, which are recycled to the first polymerization reactor, amount to 5 to 70% by weight of the recovered suspension medium. 3. Process according to claim 1 or 2, characterized in that an ethylene homopolymer is prepared in the first polymerization reactor and an ethylene copolymer is prepared in a subsequent polymerization reactor. 4. Process according to any one of claims 1 to 3, characterized by the fact that the difference between the normal boiling point of one of the comonomers fed to the reactor cascade and the normal boiling point of the diluent or, if the diluent is a mixture of components, the difference between the normal boiling point of one of the comonomers fed to the reactor cascade and the initial normal boiling point or the final normal boiling point of the diluent is not greater than 15°C, with the Normal boiling points are defined as boiling points at 1013.25 hPa. 5. Process according to any one of claims 1 to 4, characterized in that the multimodal ethylene copolymer is an ethylene-1-hexene copolymer. 6. Process according to any one of claims 1 to 4, characterized in that the multimodal ethylene copolymer comprises at least two comonomers. 7. Process according to claim 6, characterized in that the multimodal ethylene copolymer comprises at least 1-hexene and 1-butene as comonomers. 8. Process according to any one of claims 1 to 7, characterized in that a part of the recovered suspension medium is recycled directly to a subsequent polymerization reactor. 9. Process according to any one of claims 1 to 8, characterized in that the process for producing the purified part of the recovered suspension medium includes a step of evaporating a part of the recovered suspension medium and then recondensing the evaporated part of the suspension medium. 10. Process according to any one of claims 1 to 9, characterized in that the process for producing the purified part of the recovered suspension medium includes a distillation step. 11. Process according to any one of claims 1 to 10, characterized in that the process for producing the purified components of the recovered suspension medium includes a wax removal step. 12. Process according to any one of claims 1 to 11, characterized by the fact that the suspension of polyethylene particles removed from the first polymerization reactor of the reactor cascade is fed to a separator, with a part of the suspension medium being separated from the suspension and recycled to the first polymerization reactor of the reactor cascade, and a concentrated suspension of polyethylene particles is transferred to the next polymerization reactor of the reactor cascade. 13. Process according to any one of claims 1 to 12, characterized by the fact that the reactor cascade comprises at least 3 polymerization reactors and the suspension of polyethylene particles removed from the second polymerization reactor is fed to a separator, being that a portion of the suspension medium is separated from the suspension and recycled to the second polymerization reactor, and a concentrated suspension of polyethylene particles is transferred to the third polymerization reactor. 14. Apparatus for polymerizing olefinic monomers in suspension in a cascade of reactors, characterized by the fact that it comprises: - at least two polymerization reactors connected in series forming a cascade of reactors, - a separator to separate the suspension into polyolefin particles and a recovered suspension medium, - transfer lines for transferring a suspension of polyolefin particles in a suspension medium from a polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade and from the last reactor of polymerization from the reactor cascade to the separator, and - a recycling line for recycling portions of the recovered suspension medium to the first polymerization reactor of the reactor cascade, wherein the recycling line for recycling portions of the recovered suspension medium to The first polymerization reactor of the reactor cascade is equipped with a catalytic hydrogenation unit. 15. Apparatus according to claim 14, characterized in that the transfer line for transferring the polyolefin particle suspension from the first polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade is equipped with a separator for separating a portion of the suspension medium from the suspension transferred from the first polymerization reactor of the reactor cascade to the next polymerization reactor of the reactor cascade, the apparatus further comprising a recycling line for recycling the medium of suspension separated from the suspension in the separator installed between the first polymerization reactor of the reactor cascade and the next polymerization reactor of the reactor cascade to the first polymerization reactor of the reactor cascade.